1. Field of the Invention
The present invention relates generally to interference measurement, and more specifically, to an interference measurement method and apparatus of a terminal in a mobile communication system including an evolved Node B (eNB) having a plurality of transmit antennas for Multiple-Input Multiple-Output (MIMO) transmission.
2. Description of the Related Art
To meet the demand for wireless data traffic having increased since deployment of 4G communication systems, efforts have been made to develop an improved 5G or pre-5G communication system. Therefore, the 5G or pre-5G communication system is also called a ‘Beyond 4G Network’ or a ‘Post LTE System’. The 5G communication system is considered to be implemented in higher frequency (mmWave) bands, e.g., 60 GHz bands, so as to accomplish higher data rates. To decrease propagation loss of the radio waves and increase the transmission distance, the beamforming, massive multiple-input multiple-output (MIMO), Full Dimensional MIMO (FD-MIMO), array antenna, an analog beam forming, large scale antenna techniques are discussed in 5G communication systems. In addition, in 5G communication systems, development for system network improvement is under way based on advanced small cells, cloud Radio Access Networks (RANs), ultra-dense networks, device-to-device (D2D) communication, wireless backhaul, moving network, cooperative communication, Coordinated Multi-Points (CoMP), reception-end interference cancellation and the like. In the 5G system, Hybrid FSK and QAM Modulation (FQAM) and sliding window superposition coding (SWSC) as an advanced coding modulation (ACM), and filter bank multi carrier (FBMC), non-orthogonal multiple access (NOMA), and sparse code multiple access (SCMA) as an advanced access technology have been developed.
The Internet, which is a human centered connectivity network where humans generate and consume information, is now evolving to the Internet of Things (IoT) where distributed entities, such as things, exchange and process information without human intervention. The Internet of Everything (IoE), which is a combination of the IoT technology and the Big Data processing technology through connection with a cloud server, has emerged. As technology elements, such as “sensing technology”, “wired/wireless communication and network infrastructure”, “service interface technology”, and “Security technology” have been demanded for IoT implementation, a sensor network, a Machine-to-Machine (M2M) communication, Machine Type Communication (MTC), and so forth have been recently researched. Such an IoT environment may provide intelligent Internet technology services that create a new value to human life by collecting and analyzing data generated among connected things. IoT may be applied to a variety of fields including smart home, smart building, smart city, smart car or connected cars, smart grid, health care, smart appliances and advanced medical services through convergence and combination between existing Information Technology (IT) and various industrial applications.
In line with this, various attempts have been made to apply 5G communication systems to IoT networks. For example, technologies such as a sensor network, Machine Type Communication (MTC), and Machine-to-Machine (M2M) communication may be implemented by beamforming, MIMO, and array antennas. Application of a cloud Radio Access Network (RAN) as the above-described Big Data processing technology may also be considered to be as an example of convergence between the 5G technology and the IoT technology.
Meanwhile, mobile communication systems have evolved to high-speed, high-quality wireless packet data communication systems capable of providing data and multimedia services, in addition the voice-oriented services provided by previous mobile communication systems. Recently, various mobile communication standards, such as High Speed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access (HSUPA), Long Term Evolution (LTE), and LTE-Advanced (LTE-A) defined in 3rd Generation Partnership Project (3GPP), High Rate Packet Data (HRPD) defined in 3rd Generation Partnership Project-2 (3GPP2), and 802.16 defined by the Institute of Electrical and Electronics Engineers (IEEE), have been developed to support high-speed, high-quality wireless packet data communication services. In particular, LTE is a communication standard developed to support high speed packet data transmission and to maximize the throughput of the radio communication system with various radio access technologies. LTE-A is the evolved version of LTE designed to improve the data transmission capability.
Typically, LTE base stations and terminals are based on 3GPP Release 8 or 9 while LTE-A base stations and terminals are based on 3GPP Release 10. The 3GPP standard organization is preparing for the next release for more improved performance beyond LTE-A. Herein, the terms ‘base station’ and ‘eNB’ are used interchangeably. The existing 3rd and 4th generation wireless packet data communication systems (such as HSDPA, HSUPA, HRPD, and LTE/LTE-A) adopt Adaptive Modulation and Coding (AMC) and Channel-Sensitive Scheduling techniques to improve the transmission efficiency. AMC allows the transmitter to adjust the data transmission amount (i.e., the amount of data to be transmitted) according to the channel condition. More specifically, the transmitter is capable of decreasing the data transmission amount for poor channel conditions, so as to maintain the received signal error probability at a certain level, and is further capable of increasing the data transmission amount for good channel conditions so as to transmit large amount of information efficiently while maintaining the received signal error probability at an intended level. Meanwhile, the channel sensitive scheduling allows the transmitter to selectively provide services to a user communicating under good channel conditions, from among a plurality of users, so as to increase the system capacity, in contrast to allocating a channel fixedly to serve a single user. This increase in system capacity is referred to as multi-user diversity gain. Both the AMC and channel sensitive scheduling are methods of adopting the best modulation and coding scheme at the most efficient time based on the partial channel status information feedback from the receiver.
When using AMC along with a Multiple Input Multiple Output (MIMO) transmission scheme, it may be necessary to take a number of spatial layers and ranks for transmitting signals into consideration. In this case, the transmitter determines the optimal data rate in consideration of the number of layers for use in MIMO transmission as well as coding rate and modulation scheme.
Recently, research has been conducted to replace Code Division Multiple Access (CDMA) used in the legacy 2nd and 3rd mobile communication systems with Orthogonal Frequency Division Multiple Access (OFDMA) for the next generation mobile communication system. The 3GPP and 3GPP2 are in the process of standardizing an OFDMA-based evolved system. OFDMA is expected to provide superior system throughput as compared to the CDMA. One of the main factors that allow OFDMA to increase system throughput is the frequency domain scheduling capability. As channel sensitive scheduling increases the system capacity using the time-varying channel characteristic, OFDMA can be used to obtain more capacity gain using the frequency-varying channel characteristic.
FIG. 1 is a graph illustrating time-frequency resources in LTE/LTE-A system.
As shown in FIG. 1, a radio resource for transmission from an evolved Node B (eNB) to a User Equipment (UE) is divided into Resource Blocks (RBs) in the frequency domain and subframes in the time domain.
In the LTE/LTE-A system, an RB generally consists of 12 consecutive carriers and has a bandwidth of 180 kHz. Meanwhile, a subframe generally includes 14 OFDM symbols and spans 1 msec. The LTE/LTE-A system allocates resources for scheduling in units of subframes in the time domain, and in units of RBs in the frequency domain.
FIG. 2 is a diagram illustrating a time-frequency resource corresponding to 1 subframe and 1 RB as a smallest scheduling unit in downlink in an LTE/LTE-A system.
The radio resource depicted in FIG. 2 is of one subframe in the time domain and one RB in the frequency domain. The radio resource includes 12 subcarriers in the frequency domain and 14 OFDM symbols in the time domain, i.e. 168 unique frequency-time positions. In LTE/LTE-A, each frequency-time position is referred to as Resource Element (RE).
The radio resource structured as shown in FIG. 2 can be configured to transmit different types of signals as follows.
1. Cell-specific Reference Signal (CRS): A reference signal is broadcast within a cell at every subframe for use, at all the UEs within the cell, in channel estimation between the eNB and UE, monitoring radio link for validity, and fine tuning of time or frequency at baseband.
2. Demodulation Reference Signal (DMRS): A reference signal is transmitted to a specific UE for use in channel estimation to recover the information carried by Physical Downlink Shared Channel (PDSCH). A DMRS port is precoded along with the PDSCH layer connected thereto for transmission. In order to receive specific layer of PDSCH, the UE receives the DMRS port connected to the corresponding layer for channel estimation and then recovers the information carried on the corresponding layer based on the estimation result.
3. Physical Downlink Shared Channel (PDSCH): A downlink data channel used by the eNB to transmit data to the UE and mapped to REs not used for reference signal transmission in data region of FIG. 2
4. Channel Status Information (CSI)-Reference Signal (RS) (CSI-RS): A reference signal transmitted to the UEs within a cell and used for channel state measurement. Multiple CSI-RSs can be transmitted within a cell.
5. Zero Power CSI-RS (ZP-CSI-RS): A CSI-RS position at which no signal is transmitted
6. Interference Measurement Resource (IMR): CSI-RS positions and one or more of REs A, B, C, D, E, F, G, H, I, and J in FIG. 2 can be configured as IMR. The UE performs interference measurement under the assumption that all the signals received at the REs configured as IMR are interferences.
7. Other control channels (Physical Hybrid-ARQ Indicator Channel), PCFICH (Physical Control Format Indicator Channel), PDCCH (Physical Downlink Control Channel (PHICH), Physical Control Format Indicator Channel (PCFICH), Physical Downlink Control Channel (PDCCH): These channels are used to provide control information that is necessary for the UE to receive PDCCH and to transmit HARQ ACK/NACK corresponding to uplink data.
In FIG. 2, the CSI-RS can be transmitted at some of the positions marked by A, B, C, D, E, F, G, H, I, and J according to the number of antennas transmitting CSI-RS. Also, the zero power CSI-RS (muting) can be mapped to some of the positions A, B, C, D, E, F, G, H, I, and J. The CSI-RS can be mapped to 2, 4, or 8 REs according to the number of the antenna ports for transmission. For two antenna ports, half of a specific pattern is used for CSI-RS transmission; for four antenna ports, entire of the specific pattern is used for CSI-RS transmission; and for eight antenna ports, two patterns are used for CSI-RS transmission. Meanwhile, the zero power CSI-RS (muting) is always transmitted by pattern. That is, although the muting may be applied to a plurality of patterns, if the muting positions do not match CSI-RS positions, the muting cannot be applied to a part of one pattern. However, if the CSI-RS positions match the zero power CSI-RS (muting) positions, the muting can be applied a part of one pattern.
In FIG. 2, A, B, C, D, E, F, G, H, I, and J may be selectively configured as IMR. When configuring IMR to a specific UE, the UE assumes that the signals received at the REs corresponding to IMR are interference signals. The eNB configures IMR in order for the UE to measure interference strength. More specifically, the UE measures the signal strength at the REs belonging to the IMR configured thereto and regards the signal strength as interference strength.
FIG. 3 is a diagram illustrating radio resource structures of two different eNBs for explaining the concept of IMR.
Referring to FIG. 3, eNB A configures IMR C at some REs assigned to the UE located within cell A. Meanwhile, the eNB B configures IMR J at some REs assigned to UE located within cell B.
The UE located within cell A reports channel status information to the eNB A to receive PDSCH. The UE must measure (noise strength: interference strength: signal energy) on the channel to generate the channel status information. IMR aims to enable the UE to measure interference and noise strength.
If eNB A and eNB B transmit signals simultaneously, they cause interference with respect to each other. More specifically, the signal transmitted by eNB B acts as interference to the UE that receives the signal transmitted by eNB A. Likewise, the signal transmitted by eNB A acts as interference to the UE that receives the signal transmitted by eNB B.
In FIG. 3, the eNB A configures the IMR C to the UE located within cell A in order for the UE to measure the interference caused by the eNB B. The eNB A does not transmit any signal at the IMR C. As a consequence, the signal received by the UE located within the cell A at the IMR C is the signal transmitted by the eNB B, as denoted by reference numbers 300 and 310. More specifically, the UE located within the cell A receives only the signal transmitted by the eNB B and the UE can measure the received signal strength from the eNB B and determine the interference strength caused by the eNB B. Likewise, the eNB B configures the IMR J to the UE located within cell B in order for the UE to measure the interference caused by the eNB A. The eNB B does not transmit any signal at the IMR J. As a consequence, the signal received by the UE located within the cell B at the IMR J is the signal transmitted by the eNB A, as denoted by reference numerals 320 and 330.
By configuring IMR as shown in FIG. 3, it is possible to measure the interference strength caused by other eNBs or transmission points. More specifically, the IMR enables measurement of the strength of interference caused by neighbor cells or transmission points efficiently in a Multi-cell wireless communication system including a plurality of cells or a distributed antenna system. However, using the IMR is not efficient for measuring the strength of Multiuser Multiple-Input Multiple-Output (MU-MIMO) interference.
The LTE system supports MIMO transmission using a plurality of transmit/receive antennas. The MIMO transmission is a technique of multiplexing the information to be transmitted spatially in match with the instantaneous channels formed with a plurality of transmit/receive antennas. Since the MIMO transmission is performed by multiplexing a plurality of data streams spatially on one time-frequency resource, the data rate increases multiple times in comparison to the legacy non-MIMO transmission. LTE Release 11 supports MIMO transmission between up to 8 transmit antennas and up to 8 receive antennas. In this case, up to 8 data streams can be multiplexed spatially such that the maximum data rate increases 8 times in comparison to the legacy non-MIMO scheme.
Typically, MIMO transmission is classified into one of Single User-MIMO (SU-MIMO) in which multiple spatially-multiplexed data streams are transmitted to one UE and Multiuser-MIMO (MU-MIMO) in which multiple spatially-multiplexed data streams are transmitted to a plurality of UEs. In a SU-MIMO mode, the spatially multiplexed data streams are transmitted to one UE. Meanwhile, in a MU-MIMO mode, the spatially multiplexed data streams are transmitted to multiple UEs. In the MU-MIMO mode, the eNB transmits a plurality of data streams, and each UE receives at least one of the plurality of data streams transmitted by the eNB. Using MU-MIMO is advantageous, especially when the number of transmit antennas of the eNB is greater than the number of receive antennas of the UE. In SU-MIMO transmission, the maximum number of data streams that can be multiplexed spatially is limited to min (a number of transmit antennas of an eNB (NTx)), a number of receive antennas of a UE (NRx)). In the MU-MIMO transmission, the maximum number of data streams that can be multiplexed spatially is limited to min (NTx, (the number of UEs (NMS)*NRx). The IMR configuration shown and described with reference to FIG. 3 is advantageous with respect to measuring the interference strength caused by other eNBs or transmission points efficiently but disadvantageous with respect to measuring the strength of MU-MIMO interference occurring in the same eNB or transmission point.
Typically, the signal received at a UE in a multi-cell mobile communication system can be expressed by an equation as follows.
                              ∑          i                ⁢                                  ⁢                              ∑                          j              ∈                              C                i                k                                                                                    ⁢                                          ⁢                                    P                              i                ,                j                            k                        ·                          h                              i                ,                j                            k                        ·                          s                              i                ,                j                            k                                                          (        1        )            
In Equation (1), Pi,jk denotes the transmit power which the ith eNB or transmission point assigns for the jth UE in the kth subframe. hi,jk denotes a result of combining the radio channels between the ith eNB or transmission point and the jth UE and the antenna precoding for MIMO transmission. si,jk denotes the signal transmitted from the ith eNB or transmission point to the jth UE in the kth subframe. Cik denotes a set of UEs to which the ith eNB or transmission point allocates downlink resource in the kth subframe. If the number of UE included in Cik is 1, the kth eNB or transmission point transmits the signal in the SU-MIMO mode. In view of the 0th UE of the 0th eNB, equation (1) can be rewritten as follows.
                                          P                          0              ,              0                        k                    ·                      h                          0              ,              0                        k                    ·                      s                          0              ,              0                        k                          +                              ∑                                          j                ∈                                  C                  i                  k                                                            j                ≠                0                                                                                    ⁢                                    P                              i                ,                j                            0                        ·                          h                              i                ,                j                            0                        ·                          s                              i                ,                j                            0                                      +                              ∑                          i              ≠              0                                ⁢                                          ⁢                                    ∑                              j                ∈                                  C                  i                  k                                                                                                  ⁢                                          P                                  i                  ,                  j                                k                            ·                              h                                  i                  ,                  j                                k                            ·                              s                                  i                  ,                  j                                k                                                                        (        2        )            
In Equation (2), P0,0k·h0,0k·s0,0k denotes the signal component transmitted from the 0th eNB to the 0th UE, and
      ∑          i      ≠      0        ⁢          ⁢            ∑              j        ∈                  C          i          k                                          ⁢                  P                  i          ,          j                k            ·              h                  i          ,          j                k            ·              s                  i          ,          j                k            denotes the interference component caused by other eNBs. The interference components caused by other eNBs
      ∑          k      ≠      0        ⁢          ⁢            ∑              i        ∈                  C          i          k                                          ⁢                  P                  i          ,          j                k            ·              h                  i          ,          j                k            ·              s                  i          ,          j                k            can be measured using the IMR configured as shown in FIG. 3. The signal
      ∑                  j        ∈                  C          i          k                            j        ≠        0              ⁢            P              i        ,        j            0        ·          h              i        ,        j            0        ·          s              i        ,        j            0      that the 0th eNB transmits to UEs other than the 0th UE acts as MU-MIMO interference to the 0th UE that receives data from the corresponding eNB. The MU-MIMO interference cannot be measured using the IMR.
It is impossible to measure MU-MIMO interference with IMR, because the eNB incurring the MU-MIMO does not transmit any signal on IMR. Returning to reference to FIG. 3, the eNB A 320 that transmits signals to a plurality of UEs mutes at the IMR C. In this case, a UE that determines channel status information on the downlink of eNB A 320 can measure the interference incurred by the eNB B 350 at the IMR C, but cannot measure the MU-MIMO interference incurred in the eNB A 320.
If the eNB performs MU-MIMO transmission to a plurality of UEs in a state in which the target UE cannot measure the MU-MIMO interference accurately to determine the channel status information, it is difficult to obtain optimized system performance, because the eNB cannot perform link adaptation effectively. Link adaptation is a technique of allocating data rate in adaptation to the channel condition of the UE and, in the mobile communication system such as LTE, link adaptation is performed based on the channel status information transmitted by the UE. If the UE fails in measuring MU-MIMO interference and thus the channel status information transmitted to the UE is not appropriate for MU-MIMO operation, this failure makes it difficult to perform effective link adaptation.
The performance degradation occurring due to the failure to reflect the influence of the MU-MIMO interference to the channel station information is significant, especially in a mobile communication system performing the MU-MIMO transmission to a plurality of UEs, such as Massive MIMO or Full Dimension MIMO (FD-MIMO) system.
In a Massive MIMO or Full Dimension MIMO system, an eNB is provided with a few dozen or a few hundred transmit antennas. In order to improve the system performance, it is necessary to increases the number of data streams to be multiplexed, in contrast to the legacy LTE system. The mobile communication system supporting the FD-MIMO is capable of transmitting signals to a plurality of UEs simultaneously in the MU-MIMO transmission mode to achieve the above aim.
FIG. 4 is a diagram illustrating an eNB supporting FD-MIMO transmission.
Referring to FIG. 4, an eNB includes a set of a plurality of transmit antennas 400 and transmits signals to a plurality of UEs using respective transmit antennas 410, as denoted by reference numerals 420 and 430.
In FIG. 4, the transmit antennas 400 are configured in the form of a 2-Dimensional (2D) antenna array panel, and individual antennas are arranged at an interval corresponding to a function of wavelength, as denoted by reference number 410. The eNB performs high order MU-MIMO transmission to a plurality UEs. High order MU-MIMO is a technique of allocating spatially distributed transmission beams to a plurality of UEs to transmit data, using the plurality of transmit antennas of the eNB. The high order MU-MIMO transmission is performed on the same time-frequency resource, so as to dramatically improve the system throughput.
FIG. 5 is a diagram illustrating downlink transmission at an eNB and uplink transmission of the channel status information at a UE in the time domain in a conventional system.
Referring to FIG. 5, the downlink transmission of the eNB includes DL subframes with IMR 500, 520 and 550, DL subframes with CSI-RS 510, 530, and 560, and DL a subframe with aperiodic CSI trigger 540; and the uplink transmission of the UE includes UL subframes with periodic CSI 570 and 580 and an uplink subframe with aperiodic CSI 590.
As shown in FIG. 5, the eNB configures the frame, such that the IMR is transmitted in subframes 500, 520, and 550 at a regular interval. More specifically, the eNB instructs the UE to measure interference on the IMR in the corresponding subframes through high layer signaling. If the instruction is received, the UE measures interference on the corresponding IMR to generate channel status information. The eNB also transmits CSI-RS in the subframes 510, 530, and 560 and notifies the UE of this transmission through higher layer signaling. If the notification is received, the UE receives CSI-RS in the corresponding subframes to generate channel status information. Typically, the UE measures
      E    s              N      o        +          I      o      to generate the channel status information (No: strength of noise, Io: strength of interference, and Es: signal energy). The UE measures the noise strength No and the interference strength Io with IMR and the signal energy Es with CSI-RS. In FIG. 5, the UE generates the channel status information using the noise and interference strength measured on the IMR and the signal energy measured with the CSI-RS. The channel status information is classified into one of periodic channel status information that the UE reports periodically and aperiodic channel status information that the UE reports in response to a request from the eNB. The period channel status information is reported periodically at an interval configured through higher layer signaling from the eNB. The aperiodic channel status information is the channel status information which the UE reports to the eNB only when the eNB requests the UE for channel information using an aperiodic feedback indicator included in the Downlink Control Information (DCI) for Uplink Data Scheduling of the corresponding UE.
In LTE Release 11, the aperiodic feedback indicator is 1-bit or 2-bit information included in the UL DCI format 0 or DCI format 4. When using the 1-bit feedback indicator, if the aperiodic feedback indicator is set to ON, the UE transmits the channel information indicating ‘serving cell c’ to the eNB through aperiodic PUSCH feedback. Here, transmitting channel information of ‘serving cell c’ is used to indicate the downlink Component Carrier (CC) carrying DCI in the Carrier Aggregation (CA) situation. When using the 2-bit feedback indicator, the UE performs the aperiodic feedback, as defined in Tables 1-1 and 1-2.
TABLE 1-1aperiodic feedback method using 2-bit aperiodic feedbackindicator (CSI Request Field) in Transmission Mode 10Value of CSI Request FieldDescription‘00’No aperiodic CSI report is triggered‘01’Aperiodic CSI report is triggered for a set of CSIprocess(es) configured by higher layers for serving cell c‘10’Aperiodic CSI report is triggered for a 1st set of CSIprocess(es) configured by higher layers‘11’Aperiodic CSI report is triggered for a 2nd set of CSIprocess(es) configured by higher layers
TABLE 1-2aperiodic feedback method using 2-bit aperiodic feedbackindicator (CSI Request Field) in Transmission Modes 1-9Value of CSI Request FieldDescription‘00’No aperiodic CSI report is triggered‘01’Aperiodic CSI report is triggered for serving cell c‘10’Aperiodic CSI report is triggered for a 1st set of servingcells configured by higher layers‘11’Aperiodic CSI report is triggered for a 2nd set of servingcells configured by higher layers
In Tables 1-1 and 1-2, ‘serving cell c’ denotes the downlink CC linked to the uplink CC which the Carrier Indication Field (CIF) included in the DCI for Uplink Scheduling, unlike the 1-bit aperiodic feedback indicator. More specifically, if the aperiodic feedback indicator set to ‘01’ is received, the UE transmits the feedback information about the downlink CC linked to the uplink CC indicated by the CIF. If the received aperiodic feedback indicator is set to ‘10’ or ‘11,’ the UE transmits the feedback information about the downlink CC configured through higher layer signaling in association with the uplink CC indicated by the CIF.
In FIG. 5, the channel status information that the UE reports to the eNB, as denoted by reference number 570 and 580 is the periodic channel status information. The UE measures the signal energy and the noise and interference strength at the respective CSI-RS and IMR positions to generate the channel status information transmitted as denoted by reference number 570 and 580. Also, the UE measures the signal energy and the noise and interference strength at the CSI-RS and IMR positions to generate the aperiodic channel status information transmitted, as denoted by reference number 590. In conventional technology, the UE cannot reflects the MU-MIMO interference to the periodic and aperiodic channel status information so as to cause performance degradation in the system operating based on the high order MU-MIMO such as FD-MIMO system.
The MU-MIMO interference may change in size and other characteristics according to the combination of the UEs for the MU-MIMO transmission.
FIG. 6 is a diagram illustrating subframes transmitted by the eNB in the MU-MIMO transmission mode.
Referring to FIG. 6, the eNB may transmit a signal precoded with wki,j at the transmit power of Pki,j to a set Cik of UEs including UE j in the kth subframe.
FIG. 6 shows that the eNB performs MU-MIMO transmission to different combinations of UEs in each subframe. For example, the eNB i performs MU-MIMO transmission to the UEs included in the set Ci0 at subframe 0. Meanwhile, the eNB i performs MU-MIMO transmission to the UEs included in the set Ci1 at subframe 1. The UEs included in a certain set Cik at a certain subframe k are determined by the scheduler of the eNB, and may change at each sub frame. Whenever the combination of the UEs changes, the signals to be transmitted to the UEs and UE-specific precodings are changed. The precoding is applied to optimize the weights of the antennas for transmitting the signals to the UEs efficiently. One representative example of precoding is to form a beam in a direction to a specific UE by applying weights to a plurality of antennas.
FIG. 7 is a conceptual diagram illustrating a concept of MU-MIMO interference to a UE when the eNB performs MU-MIMO transmission to a plurality of UEs in a subframe.
Referring to FIG. 7, UEs A, B, C, and D receive a precoded PDSCH signals transmitted by an eNB through radio channels in forms denoted by reference numbers 700, 710, 720, and 730.
The UE A receives the signal transmitted by the eNB in the form as denoted by reference number 700. In FIG. 7, Pi,Ak·hi,Ak·si,Ak denotes the signal received at the UE A as a result that the PDSCH signal precoded by the eNB propagates through a radio channel between the eNB and the UE A. In Pi,Ak·hi,Ak·si,Ak, hi,Ak denotes the precoding and influence of the radio channel. In FIG. 7, the UE A experiences the influence of the signals 710, 720, and 730 transmitted from the eNB to other UEs B, C, and D in receiving the signal Pi,Ak·hi,Ak·si,Ak transmitted thereto. If link adaptation is performed in consideration of the strength of such interferences, it is difficult to attain the advantages of MU-MIMO transmission, such as FD-MIMO for the optimization of the system throughput in a mobile communication system.
When using the high order MU-MIMO such as FD-MIMO, it is also important to consider the number of UEs to be scheduled simultaneously. When the eNB operates in the MU-MIMO transmission mode, the number of target UEs as well as the combination of the target UEs varies at every subframe. More specifically, the number of UEs to which the eNB transmits signals in the MU-MIMO mode at the subframe 600 may differ from the number of UEs to which the eNB transmits data at subframe 610.
Typically, the eNB performs downlink transmission at a limited transmit power. Assuming that the maximum allowed transmit power of the eNB is Ptotal, Ptotal is divided into the number of UEs for MU-MIMO transmission thereto. In order to accomplish this efficiently, it is necessary for the UE to know the transmit power allocated by the eNB for transmission to the UE. If the UE has no such information, it cannot determine the data rate for receiving downlink data, resulting in degradation of system throughput.
FIG. 8 is a conceptual diagram illustrating the transmit powers allocated for respective UEs and the transmit power of CSI-RS for the UE to generate channel status information when the eNB transmits signals in the MU-MIMO mode.
In FIG. 8, the PDSCH addressed to the UE is transmitted in the MU-MIMO transmission mode. Accordingly, the transmit power of the eNB is divided into the number of UEs. Meanwhile, the transmit power for CSI-RS transmitted in order for the UE to generate channel status information is not necessarily divided. In the example of FIG. 8, if the UE generates the channel status information without awareness that the transmit power of PDSCH is ¼ of the transmit power of the CSI-RS, the reports incorrect channel status information to the eNB, resulting in degradation of MU-MIMO transmission performance.
In order to optimize the throughput of the FD-MIMO system as shown in FIG. 4, the UE must generate the channel status information to be reported to the eNB in consideration of the MU-MIMO interference occurring at the eNB transmitting PDSCH to the UE, as well as the interference incurred by other eNBs. Therefore, there is a need for a MU-MIMO interference measurement method that is capable of allowing the UE to generate accurate channel status information. Also, there is a need for a method of informing the UE of the transmit power of the eNB that is allocated for transmission to the UE in generating the channel status information.